The present disclosure relates to a fixing device for fixing a toner image transferred onto a recording medium, and an image forming apparatus such as a copier or a printer including the fixing device. In particular, the present disclosure relates to a fixing device including a separation plate for preventing the recording medium from being wrapped around a heated roller member such as a fixing roller.
In a conventional image forming apparatus of an electrophotographic method, an image carrier such as a photosensitive drum that is uniformly electrified by an electrifying device is irradiated with a laser beam from an exposing device, and hence a predetermined electrostatic latent image is formed in which charge is partially attenuated. Then, a developing device generates a toner image by causing toner to adhere to the electrostatic latent image, transfer means transfer the toner image onto a paper sheet (recording medium), and a fixing device heats and press the unfixed toner to be a permanent image, so that an image forming process is performed.
In the fixing device described above, there is a problem that a paper sheet is wrapped around the fixing roller (heated roller member) or a pressure roller (pressure member) constituting a fixing roller pair (fixing member), resulting in jamming. It is a general method to secure paper sheet separation performance by disposing a separation member such as a separation claw or the separation plate on the downstream side of the nip portion (fixing nip portion) between the fixing member and the pressure member and by precisely controlling a gap between the tip of the separation member and the fixing member (or the pressure member). Further, in a color image forming apparatus, more toner adheres onto the paper sheet than in a monochrome image forming apparatus, and hence melted toner is easily adhered to the fixing member. For this reason, a tube or a coat layer of fluorine-based resin such as PFA is usually disposed as a releasing layer on a surface of the fixing member.
When the above-mentioned releasing layer is disposed on the surface of the fixing member, there is a problem that poor transfer or electrostatic offset easily occurs when a paper sheet containing excessive moisture under a high temperature and high humid environment is fed to the fixing nip portion.
There is known a fixing device provided with transfer member detection means including an actuator that can rotate about a rotation axis on a downstream side in a transport direction of a transfer member (paper sheet) and contacts with the transfer member so as to detect passing of the transfer member, and a conductive member rotating integrally with the actuator. In this fixing device, the conductive member rotates to be away from the ground electrode and the surface of the pressure roller when the passing of the transfer member is not detected by the actuator, while the conductive member rotates to contact with the ground electrode and the surface of the pressure roller when the passing of the transfer member is detected by the actuator. In this way, it is possible to prevent electrostatic offset due to charge accumulated on the pressure roller by bringing the conductive member to contact with pressure roller.